Means for conducting electricity in various products such as a semiconductor product, an electrical product, an electronic product, a solar battery, and an automobile can be a method of soldering or making contact using a terminal formed with a conductive base.
For such a terminal as described above, in order to improve solderability or corrosion resistance of the surface of the conductive base, the surface is usually covered with a metal such as Au, Ag, Pd, Cu, Ni, In, Sn and Sn—Pb alloy (Japanese Patent Laying-Open No. 1-298617 (Patent Document 1) for example). Among these metals, Sn and Sn—Pb alloy are most commonly used in consideration of the cost and the like, and the electroplating method is usually adopted as a method of covering.
When the electroplating is performed with Sn alone, however, a large columnar crystal is generated in the thin covering film, which promotes generation of a whisker. Since generation of the whisker causes an electrical short circuit, the generation thereof is required to be prevented.
As means for preventing the generation of the whisker, alloying of Sn, that is, use of an Sn—Pb alloy or the like has been conventionally attempted. Since Pb is a toxic metal as is well-known, however, use thereof is limited from environmental considerations. Further, when the Sn—Pb alloy is used for such a product as automobile where considerable vibrations occur, the surface could be oxidized due to rubbing to cause an increase in contact resistance and thereby cause an undesirable phenomenon that is so-called fretting wear.
Therefore, attempts have been made to develop methods of forming various Sn-based alloys by means of electroplating as substitutes for the Sn—Pb alloy and to use these alloys in combination with a layer plated with Sn alone (Japanese Patent Laying-Open No. 10-229152 (Patent Document 2), Japanese Patent Laying-Open No. 2003-342784 (Patent Document 3)). Regarding an Sn—Cu alloy for example, although it has a minimum melting point (227° C.) and shows good solderability with 99.3 mass % of Sn and 0.7 mass % of Cu, generation of the whisker (columnar crystal) cannot be effectively prevented because of the small content of Cu. In contrast, if the content of Cu is increased, the melting point significantly increases and thus the solderability is deteriorated.
The Sn-based alloy is sometimes used in melting solder such as solder dip or cream solder only for adhering the terminal as mentioned above. As such an Sn-based alloy, an alloy made of Sn, Ag and Cu is used in some cases (Japanese Patent Laying-Open No. 5-50286 (Patent Document 4)).
The Sn-based alloy used in such a manner as described above, however, merely shows an adhesion property by heat melting (melting solder) of each metal of Sn, Ag and Cu (or an ingot obtained by melting and mixing these metals), and an application thickness thereof cannot be controlled. Therefore, on the base material, a uniform coating of a thin film having a small thickness of at most 100 μm cannot be formed.
If the uniform coating of the thin film having a small thickness cannot be formed, lack of stability in appearance properties occurs and an electrical short circuit is caused. Furthermore, a pinhole or the like is easily generated to cause deterioration in corrosion resistance.
In attempt to solve the above-described problems, a proposal has been made concerning such a terminal as connector plated with a tin-silver-copper ternary alloy (Japanese Patent Laying-Open No. 2001-164396 (Patent Document 5)). A method shown by this proposal, however, is characterized by a plating bath containing a specific sulfur compound to prevent a copper compound in the plating bath from being deposited on a tin electrode. The concentration of the sulfur compound, however, must be increased to increase the concentration of the copper compound in the plating bath, which could destroy the balance of components in the plating bath. Therefore, the copper compound of a high concentration cannot be used in the plating bath and the concentration of copper in the tin-silver-copper ternary alloy plating film cannot be increased, which results in a problem that a thin plating film having a low melting point cannot be obtained.
Further, a proposal has also been made regarding tin-silver-copper ternary alloy plating using water-soluble silver salt together with water-soluble tin salt and water-soluble copper salt (Japanese Patent Laying-Open No. 2001-26898 (Patent Document 6). According to the proposal, however, such a sulfur compound as thioamide compound or thiol compound is used, which results in a problem, like the aforementioned one, that a thin plating film having a low melting point cannot be obtained.
Patent Document 1: Japanese Patent Laying-Open No. 1-298617
Patent Document 2: Japanese Patent Laying-Open No. 10-229152
Patent Document 3: Japanese Patent Laying-Open No. 2003-342784
Patent Document 4: Japanese Patent Laying-Open No. 5-50286
Patent Document 5: Japanese Patent Laying-Open No. 2001-164396
Patent Document 6: Japanese Patent Laying-Open No. 2001-26898